1. Field of the Invention
The present invention relates to a semiconductor device including a circuit composed of a thin film transistor (hereinafter referred to as TFT) and a method of manufacturing the same. For example, the present invention relates to an electro-optical device represented by a liquid crystal display panel and to an electronic equipment incorporating the above-mentioned electro-optical device as a part thereof.
2. Description of the Related Art
Note that, the term semiconductor device defined in this specification refers to all the devices which can operate by utilizing a semiconductor characteristic, and the electro-optical device, a semiconductor circuit and the electronic equipment are all included in a category of the semiconductor device.
In recent years, development of a semiconductor device has been in progress, in which the thin film transistor (TFT) is formed by using a semiconductor thin film (film thickness: approximately several to several hundreds of nm) formed on a substrate having an insulating surface and the TFT is adapted to constitute a large-area integrated circuit.
An active matrix type liquid crystal module, an EL module, and a contact image sensor are known as a typical example thereof. In particular, the TFT using a silicon film having a crystalline structure (typically, a polysilicon film) as an active layer (hereinafter, referred to as polysilicon TFT) is high in a field effect mobility and thus can be used to form a circuit having various functions.
For example, the liquid crystal module mounted onto a liquid crystal display device includes on one substrate a pixel portion and a driver circuit such as a shift register circuit, level shifter circuit, buffer circuit, or sampling circuit which is based on a CMOS circuit. The pixel portion performs an image display for each functional block and the driver circuit controls the pixel portion.
Further, in a pixel portion of the active matrix type liquid crystal module, a TFT (pixel TFT) is arranged in each of several tens to several millions of pixels and each pixel TFT is provided with a pixel electrode. An opposing electrode is provided on an opposite substrate side with a liquid crystal interposed between the two electrodes, to thereby form a kind of capacitor with the liquid crystal used as a dielectric. A voltage applied to each pixel is then controlled by a switching function of the TFT to control the application of charge to the capacitor to drive the liquid crystal, thereby displaying an image through the control of an amount of transmitted light.
The pixel TFT is composed of an n-channel TFT and adapted to drive the liquid crystal by applying a voltage as a switching element. The liquid crystal is driven with an alternating current and a method called a frame inversion driving is employed in many cases. In this method, as a required characteristic of the pixel TFT, it is important to sufficiently lower an OFF current value (a drain current caused to flow at the time of TFT being in OFF state) in order to reduce the power consumption.
Conventionally, when irradiated with a laser light for crystallization or for improvement in crystallinity, the semiconductor film is instantly melted from the surface thereof and then the melted semiconductor film due to heat conduction to the substrate is cooled to solidify from the substrate side. Through the solidifying a process, recrystallization is performed to form a semiconductor film having a crystalline structure with a large grain size. However, the semiconductor film once melted causes a volume expansion to form unevennesses called ridges on the semiconductor surface. In particular, in a case of a top gate type TFT, the surface having the ridge serves as an interface with a gate insulating film so that element characteristics are largely influenced.
In general, an excimer laser or Ar laser is used for laser annealing in many cases. A method of performing the laser annealing described below has been preferably used because it is suited for mass production with high productivity. That is, a pulse oscillation type laser beam having a high output is processed by an optical system so as to be a square spot whose side is of several centimeters or be linear with a length of, for example, 10 cm or more on a surface to be irradiated and the laser beam is scanned on the surface to be irradiated with the irradiation point relatively changed with respect to there. In particular, in a case where a linear laser beam (hereinafter, referred to as linear beam) is used in the surface to be irradiated, unlike in a case of using a spot-shaped laser beam that requires scanning vertically and laterally, it is sufficient to perform the scanning only in a direction perpendicular to a line direction of the linear beam in order to apply the laser beam to the entire surface to be irradiated, thereby achieving a high productivity. The reason the scanning is performed in the direction perpendicular to the line direction is that the direction enables the most efficient scanning. Because of the high productivity, the linear beam obtained by processing laser having a high output by an appropriate optical system has been mainly used for the laser annealing. Also, the linear beam is applied while being overlapped by gradually shifting it in a transverse direction thereof so that an entire surface of an amorphous silicon film is subjected to the laser annealing, thereby making it possible to crystallize or improve the crystallinity.
Thus, the technique of the laser annealing is indispensable in order to form a semiconductor film having higher electric characteristics in a lower cost.
However, there remain problems in the conventional crystallization by the laser light such that energy is not uniformly applied to the entire film and wavelike traces of irradiation with the laser light are remained in addition to the ridge.
Further, if the unevennesses on the film surface formed after the crystallization are leveled by using an etch back method, a CMP method or the like, the number of steps increases and at the same time the semiconductor film becomes thinner, so that it is difficult to level the surface of the semiconductor thin film having a thickness of 100 nm or less with a good controllability.